The object of this invention is to make a more flame-retardant flexible cable for use inside buildings for interconnecting telephone and other low voltage communication or signal circuits.
Basically to accomplish this objective, the usual PVC plastic jacket over the cable core has been replaced with an annealed flexible aluminum or copper tube that is made from a longitudinal strip that is formed into a tube over the core, continuously welded, drawn down snugly onto the core and annealed in a continuous operation.
The cable core, consisting of a number of insulated single or paired conductors, is wrapped with heat barrier tapes to protect the core during manufacture and subsequent use in the field. These tapes may consist of one or more layers of asbestos, paper, fiberglass, heat-resistant plastic or the like; however, paper tapes either helically or longitudinally applied are the most economical at this time; and crepe paper is advantageous for lower heat conduction.
The conductor insulator is a specially formulated semi-rigid PVC that is self-extinguishing and it emits minimum smoke fumes when exposed to fire due to the use of relatively large amounts of inorganic materials in its composition.
Some of the advantages of this invention over conventional polyvinyl chloride plastic jacketed cables are that the non-combustible metal sheath of the invention replaces a plastic jacket which comprises more than half of the combustible material weight in the conventional cable. For example, a popular size of conventional cable containing 25 pairs of No. 24 AWG wire contains 27 lbs. of PVC jacket material per thousand feet and 24 lbs. of PVC conductor insulation. Also, the non-combustible metal sheath greatly reduces flame spread in the event of a fire and also prevents afterburn in the cable of the present invention.
Elaborate tests prescribed by the Underwriters Laboratories* showed that aluminum sheathed cables of this invention had negligible flame travel or spread beyond the point of direct flame application, while all comparable plastic sheathed cables had flame spread the entire length of the cable in vertical flame tests and for 10 to 15 feet in horizontal flame tests. FNT *U.L. 1277; ASTM-E 84 (modified for cable)
It was also discovered that the intermixing of aluminum sheath cables among plastic sheath cables, which might be cables already installed, had the effect of reducing flame spread among all of the cables, as was demonstrated by tests. The metal sheath is easily removed from the core to facilitate terminating the cable.
A still further advantage is that the lightweight flexible annealed aluminum sheath cables are easy to install and such an installation is far more economical than placing plastic sheath cables in metal conduits.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.